GPSデータで見られる東海スロースリップに伴う歪の時空間変化

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Slow slip event (SSE) detected by the GPS network (GEONET) of the Geographical Survey Institute has been observed in the southern coastal area of the central Japan, the Tokai area, since 2001. Although appreciable changes corresponding to SSE have not yet been observed by the borehole strainmeters of the Japan Meteorological Agency in the area, we evaluated space and time variations of the strain caused by SSE before detecting them with the strainmeters. Use of strain has merits for evaluating a localized event. That is, i) strain does not reflect a common displacement observed in a wide area, irrespective of reference station, ii) strain is comparable directly with the outputs of borehole strainmeters, extensometers, etc. and iii) the elastic strain caused by a source diminishes rapidly compared to a displacement, for example, in inverse proportion to the cube of distance from it, which means ease in estimating the source location. According to the analysis, we could clearly detect strain changes associating with SSE. Expansion appeared in and around Hamanako-lake of the central Tokai area, and at the same time contraction appeared around Omaezaki and Atsumi in the eastern and western parts of the area, respectively. The peak of expansion is now moving eastward and steadily growing, and the contraction is gathering in intensity. The maximum principal contraction rates increased 1.5 times after the commencement of SSE. The total scalar moment estimated from the distribution of dilatation in January 2003 becomes 1.3x1019 [Nm] and the moment magnitude becomes 0.1, approximately. A high concentration of changes in Coulomb fracture stress is seen in the eastern Tokai area.

著者

吉川澄夫
気象研究所

GPSデータで見られる東海スロースリップに伴う歪の時空間変化

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